Nowadays, small speakers are broadly applied in many cases such as on-vehicle acoustic systems where there is no much space to accommodate large speakers. A passive radiator is generally used in a speaker to augment the bass performance, by tuning the design parameters correctly. When used properly, a passive radiator can give to a speaker with the comparable performance characteristics of a large speaker in which a “bass-reflex” port tube is used to augment the bass performance.
FIG. 1a is a cross-sectional side view of a typical speaker with a passive radiator included therein. As shown in FIG. 1a, the speaker 1 includes an enclosure 11, a transducer 12 and a passive radiator 13. Both the transducer 12 and the passive radiator 13 are mounted in the enclosure 11. Note that, the term “transducer” may also be referred to as “speaker driver” which is an active device for converting electricity into sound. A passive radiator looks like a normal transducer from the front. But, as suggested by its name, a passive radiator is a reactionary device and does not include a voice coil, a magnet and electronic structure attached to it, all of which are usually included in a normal transducer as well known in the art.
Specifically, the sound generated by the transducer 12 affects the internal air pressure of the enclosure 11, and the internal air pressure fluctuation caused by the transducer 12 in turn causes the passive radiator 13 to begin moving forward/backward so as to create sound frequencies as if it was also a transducer. Accordingly, by tuning properly, the passive radiator 13 can increase low frequency response (bass) of the speaker 1.
FIG. 1b further shows the correspondence between the cross-sectional side and front views of the passive radiator 13 in FIG. 1a. In FIG. 1b, the cross-sectional side view of the passive radiator 13 which is similar to that of FIG. 1a is shown on the upper side and the cross-sectional front view thereof is shown on the lower side. As shown in FIG. 1b, the passive radiator 13 includes a suspension 131 and a center panel 132 encircled by and adhered to the suspension 131. The suspension 131 provides the compliance, that is, ensures the center panel 132 vibrates in the axial direction of the passive radiator 13 but does not in the radial direction thereof. The center panel 132 provides the acoustic mass for the passive radiator 13 to work properly. The suspension 131 may be made of a soft and flexible material such as rubber, and the center panel 132 may be made of a hard material such as metal. Note that, although the center panel 132 is planar as shown in FIGS. 1a and 1b, it is only for the exemplary purpose and it may be of any other shapes, for example coned like the transducer 12.
However, as the miniaturization of speaker develops, the enclosure of speaker may not accommodate both a transducer and a passive radiator, which needs to be resolved.